In the ongoing question about whether the Bible condones human slavery, Christian apologists have come up with many ways to try and explain that it doesn't. One Christian is Glenn Miller, who wrote a piece on the Christian Think Tank website on slavery in the Bible arguing this point. To properly answer this question, one should ask whether Mosaic law allowed foreigners in Israel to be legally kept in conditions amounting to slavery.
So what is slavery? Slavery has many definitions. For example:
1. The condition in which one person is owned as property by another and is under the owner's control, especially in involuntary servitude.
2. (Law) the state or condition of being a slave; a civil relationship whereby one person has absolute power over another and controls his life, liberty, and fortune
3. The subjection of a person to another person, esp in being forced into work
All of these paint a situation one could properly call slavery. Interestingly, to be a slave does not require it to be based on race or ethnicity, and it does not have to be life long. Someone forced into servitude and labor for a finite amount of time can still be considered a slave during the time they are forced. I mention this because many Christian apologists are quick to point out that biblical slavery was not exactly like slavery in the Antebellum South. That may be so, but that doesn't mean biblical slavery wasn't slavery. In the Old Testament, Mosaic law describes how foreigners (non-Hebrews) could be forcefully taken as slaves by being acquired by war (Deut. 20:12-14) and foreign slaves could be kept for life (Lev 25:44-46). While the servitude forced upon prisoners convicted of just crimes is not generally considered slavery, this didn't apply to the people the Bible mentions were forced into servitude. So at least two conditions have to be met in order to properly be called slavery: (1) The person has to be forced into the position against their will, and (2) the person has to be made to perform some kind of labor, and paid nothing or next to nothing, for a certain amount of time, up to life. Now, we can endlessly split hairs over exactly what's "force" (does verbal intimidation or coercion count as force?), but it's not necessary now, as clearly being threatened with death at the end of a sword counts as force.
I made a post a while back called Biblical Slavery for Foreigners where I quote from A History of Ancient Near Eastern Law, a scholarly work that mentions the allowance of forced lifelong slavery for foreigners in ancient Israel. Miller's article references my source 21 times but when he quotes from it he often is quoting from slave systems of other non-Hebrew cultures. For example, his citation of page 449 references Assyrian slavery. Page 585 references Mesopotamian uses of slavery. Page 664 references Emar, part of Anatolia and the Levant. Page 741 refers to Canaanite culture. And page 199 refers to Mesopotamian culture again. None of these references refer to Hebrew culture and law which is the very thing in question. We're not debating what the Sumerians did or the Hittites did. We're debating what the Israelites did because Christians believe their law came from Yahweh—the one true god, and many Christians today are still claiming this god — and only this god — grounds morality. That's what the debate is about. And all these points Miller makes that slavery was sometimes (or even often) an economic need is totally irrelevant. No one denies that indentured servitude existed in the ANE. When debating whether the Bible condones slavery we're having an in principle argument here: did Mosaic law condone forced servitude that could last for life under any circumstances? Yes or no? That is the issue. Showing that most slavery was voluntary indentured servitude in the ANE is totally irrelevant.